Tying a new retaining wall into an existing concrete foundation is a common task in NZ residential landscaping, but it needs to be done correctly to ensure structural integrity and compliance with the NZ Building Code. The method typically involves drilling into the existing concrete and installing epoxy-anchored reinforcing bars (rebar) or mechanical anchors to create a solid connection between old and new work. Get this wrong and you risk wall failure, soil movement, or costly remediation — so understanding the process (and its limits) is essential before you pick up a drill.
Understand the Consent Requirements First
Before any work begins, check whether your retaining wall requires a building consent. Under the Building Act 2004 and its Schedule 1 exemptions, retaining walls that retain less than 1.5 metres of ground and do not support a surcharge (such as a driveway, vehicle load, or structure) may be exempt from consent — but this varies by council and site conditions. Walls over 1.5 m, walls on slopes, or walls near boundaries almost always require consent.
Always confirm with your local council before starting. You can find general guidance on exemptions at MBIE Building Performance. BRANZ also publishes useful technical guidance on retaining wall construction.
Assess the Existing Foundation
Before drilling, you need to understand what you're connecting to:
- Confirm the foundation is structurally sound. Cracked, crumbling, or poorly compacted concrete may not be suitable as an anchor point without remediation.
- Check the thickness and reinforcement. Ideally, the existing slab or footing is at least 150–200 mm thick and contains rebar. If you're unsure, a structural engineer can advise.
- Locate existing rebar using a rebar locator (cover meter) to avoid drilling through it unintentionally.
- Check for services — buried pipes or conduits can run through or under slabs. Contact Dial Before You Dig if there's any doubt.
Tools and Materials You'll Need
| Item | Purpose |
|---|---|
| SDS rotary hammer drill | Drilling into concrete |
| Diamond-tipped or carbide core bits | Clean, accurate holes |
| Rebar (typically 12–16 mm HD) | Structural connection |
| Epoxy anchoring adhesive (e.g. Sika AnchorFix) | Bonding rebar into drilled holes |
| Wire brush and compressed air | Hole preparation |
| PPE (safety glasses, dust mask, gloves) | Personal protection |
Products like Sika AnchorFix or similar structural epoxy adhesives are widely used in NZ for this application. Always follow the manufacturer's data sheet for cure times, hole diameter, and embedment depth — these are critical to achieving the rated load capacity.
Step-by-Step: Drilling and Anchoring Rebar
1. Mark Your Anchor Positions
Layout the rebar positions to match your new wall's reinforcement spacing — typically 400–600 mm centres horizontally, depending on your engineer's specification or the wall design.
2. Drill the Holes
Use a rotary hammer drill with the correct diameter bit (usually 2–4 mm larger than the rebar diameter). Drill to the required embedment depth — this is specified by the epoxy manufacturer and is typically 10–15 times the bar diameter (e.g. a 12 mm bar may need 120–180 mm embedment).
3. Clean the Holes Thoroughly
This step is critical and often skipped. Use a wire brush and compressed air (or a hand pump) to remove all dust and debris. Dirty holes dramatically reduce epoxy bond strength.
4. Inject the Epoxy
Follow the manufacturer's instructions exactly. Most cartridge systems require you to dispense and discard a small amount first to ensure proper mixing, then fill the hole from the bottom up to avoid air pockets.
5. Insert the Rebar
Insert the rebar with a slow rotating motion to ensure full epoxy coverage. Hold or prop it in position during the gel time — do not disturb it until fully cured. Cure times vary significantly with temperature; cold NZ winters will extend cure times.
6. Tie into New Wall Reinforcement
Once cured, the protruding rebar becomes part of your new wall's reinforcement cage, tied with bar chairs and wire before pouring new concrete.
Drainage Is Critical
Retaining walls in NZ must manage groundwater effectively to prevent hydrostatic pressure build-up behind the wall. Ensure your design includes subsoil drainage (ag pipe, gravel backfill, and weepholes) in accordance with good practice. Poor drainage is one of the leading causes of retaining wall failure in NZ's wet climate.
When to Call a Professional
Retaining walls are structural elements — failure can cause serious property damage or injury. You should engage a licensed building practitioner (LBP) or structural engineer if:
- The wall is over 1.0 m high or retains a surcharge load
- The site has unstable or expansive soils, or is on a slope
- You're unsure about the condition or capacity of the existing foundation
- A building consent is required
- The wall is near a boundary, building, or public space
You can verify LBP registration at Licensed Building Practitioners. For structural engineering input, your council's building department can advise on when an engineer's producer statement is required.
For quality local retaining wall builders in your area, search tradedirectory.co.nz to find experienced, vetted professionals.
Licensed Building Practitioner (LBP)
